Sayyiduna Malik bin Buhaina (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) saw a man who, after the takbir, was performing two rak‘ahs of prayer. When the Prophet (peace and blessings be upon him) finished the prayer, people gathered around that man, and the Prophet (peace and blessings be upon him) said: "Are you performing four rak‘ahs of the morning prayer? Has the morning prayer become four rak‘ahs?"
Hadith Referenceاللؤلؤ والمرجان / كتاب صلاة المسافرين وقصرها / 413
Hadith Gradingمحدثین:«صحیح»
Hadith Takhrij«صحیح، أخرجه البخاري في: 10 كتاب الأذان: 38 باب إذا أقيمت الصلاة فلا صلاة إلا المكتوبة»
Brief Explanation
Imam Shawkani rahimahullah states that after hearing the (opening) takbir, it is not permissible (halal) for a person performing prayer to offer the Sunnah prayers of Fajr or any other voluntary (nafl) prayer, whether he is inside the mosque or outside. If he does so, he has disobeyed Allah Ta'ala and His Messenger sallallahu alayhi wa sallam. This is also the fatwa of the Ahl al-Zahir, and Imam Ibn Hazm rahimahullah has transmitted this same ruling from Imam Shafi'i rahimahullah and the majority of the Salaf. (Raaz)